diff --git a/lib/tunnelbroker/secondary-tunnelbroker-connection.js b/lib/tunnelbroker/secondary-tunnelbroker-connection.js
--- a/lib/tunnelbroker/secondary-tunnelbroker-connection.js
+++ b/lib/tunnelbroker/secondary-tunnelbroker-connection.js
@@ -6,14 +6,14 @@
 
 export type SecondaryTunnelbrokerConnection = {
   // Used by an inactive tab to send messages
-  sendMessage: MessageToDeviceRequest => mixed,
+  +sendMessage: MessageToDeviceRequest => mixed,
   // Active tab receives messages from inactive tabs
-  onSendMessage: ((MessageToDeviceRequest) => mixed) => RemoveCallback,
+  +onSendMessage: ((MessageToDeviceRequest) => mixed) => RemoveCallback,
 
   // Active tab sets the message status of messages from inactive tabs
-  setMessageStatus: (messageID: string, error: ?string) => mixed,
+  +setMessageStatus: (messageID: string, error: ?string) => mixed,
   // Inactive tabs receive message status and resolve or reject promises
-  onMessageStatus: (
+  +onMessageStatus: (
     ((messageID: string, error: ?string) => mixed),
   ) => RemoveCallback,
 };